A correspondence object pair describes the same object of an anatomical body structure in two different reference frames. So-called anatomical landmarks may be used as the objects since they are easy for the physician to identify in the anatomical body structure. In accordance with the invention, not only landmarks but any other objects, including parts of the anatomical structure or geometric primitives that have a specific relationship to the body structure (for example, points on the surface, objects of symmetry, tangential objects, fitted geometric objects) can be used to determine correspondence object pairs and to determine the spatial relationship between the two reference frames.
Prior to performing navigation relative to a body structure, there may be a (virtual) reference frame in which an anatomical body structure is three-dimensionally spatially described, based on medical analysis methods (for example, magnetic resonance tomography (MRT) or computer tomography (CT)). These analysis methods are typically performed at an earlier point in time (for example, pre-operatively) and provide detailed information concerning the body structure. This detailed information may be used for planning an operation. It may be desirable to compare the virtual reference frame (for example, the “virtual” data concerning the anatomical body structure on which the planning of an operation is based) with obtained “actual” body structure data, for example, intra-operatively. To this end, marker devices may be attached to the body structures of the patient, for example, to the bone. When detected, the marker devices allow actual body structure data to be obtained, the terms “actual” and “virtual” are used in the following to distinguish the aforesaid reference frames and the corresponding data and objects. The positional relationship between the anatomical body structure and the marker devices may not be known at the beginning of the operation. To compare the virtual reference frame with the actual reference frame, so-called landmarks on the body structure may be (intra-operatively) touched, using a pointer. Markers may be likewise attached to the pointer with known positions relative to the tip of the pointer. Therefore, the tip of the pointer and the position of the landmark can be detected by a marker detection device. The marker detection device also may detect the marker device attached to the body structure. The detection of the body structure marker device provides the relative position between the marker device and the tip of the pointer and, therefore, the relative position between the marker device and the landmark. At least three landmarks may be captured in the actual reference frame by the surgeon via the pointer and read into a navigation system. The same (corresponding) landmarks may be identified by the physician in the virtual reference frame. Using the determined pairs of corresponding landmarks (that represent an example of correspondence object pairs) it is possible to establish the spatial relationship between the actual reference frame and the virtual reference frame. The degree of accuracy of the special relationship may depend on the relative position of the landmarks. With this relationship established, it is possible to navigate an instrument (having a marker device) relative to the anatomical body structure, wherein the position of the instrument relative to the virtual body structure data can be viewed on a screen.
In reality, determining the landmarks can be difficult. Usually, only a small region of the anatomical body structure (for example, the bone structure) is exposed, such that only a few prominent surface shapes on the body structure (that can be used as landmarks) are available. In other words, if the surface structure of the bone is not prominent, landmark determination can become uncertain.